The present invention relates to the field of computers and particularly to computers having central processing units (CPU's) that operate in accordance with the IBM ESA/390 architecture and particularly to host computers that have interpretive execution of guests.
ESA/390 architecture computers are controlled in part by a Program Status Word (PSW). The program-status word (PSW) includes the instruction address, condition code, and other information used to control instruction sequencing and to determine the state of the computer. The active or controlling PSW is called the current PSW. It governs the program currently being executed.
The CPU has an interruption capability, which permits the CPU to switch rapidly to another program in response to exception conditions and external stimuli. When an interruption occurs, the CPU places the current PSW in an assigned storage location, called the old-PSW location, for the particular class of interruption. The CPU fetches a new PSW from a second assigned storage location. This new PSW determines the next program to be executed. When it has finished processing the interruption, the interrupting program may reload the old PSW, making it again the current PSW, so that the interrupted program can continue.
The status of the CPU can be changed by loading a new PSW or part of a PSW. Control is switched during an interruption of the CPU by storing the current PSW, so as to preserve the status of the CPU, and then loading a new PSW.
A new or modified PSW becomes active (that is, the information introduced into the current PSW assumes control over the CPU) when the interruption or the execution of an instruction that changes the PSW is completed.
A storage key is associated with each 4K-byte block of storage that is available in the configuration. The storage key has the following format. ##STR1## The bit positions in the storage key are allocated as follows: Access-Control Bits (ACC):
If a reference is subject to key-controlled protection, the four access-control bits, bits 0-3, are matched with the four-bit access key when information is stored, or when information is fetched from a location that is protected against fetching. Fetch-Protection Bit (F):
If a reference is subject to key-controlled protection, the fetched protection bit, bit 4, controls whether key-controlled protection applies to fetch-type references are monitored and that fetching with any access key is permitted; a one indicates that key-controlled protection applied to both fetching and storing. No distinction is made between the fetching of instructions and of operands.